Acrylamide is used as an industrially important substance in a wide range of areas. For example, acrylamide polymers are widely used as flocculants for wastewater treatment, paper strength agents, oil recovery agents and the like. Acrylamide has been conventionally industrially produced by hydrating the corresponding acrylonitrile using reduced copper as a catalyst. Recently, however, a method using a microbial catalyst instead of a copper catalyst (biocatalyst method) has been developed and partially practically used.
Since the biocatalyst method requires only mild reaction conditions, hardly produces by-products, and allows construction of a very simple process, it has been regarded as a hopeful industrial production method, and many microorganisms having an enzyme that has the catalytic ability to convert acrylonitrile into acrylamide by hydration (enzyme name: nitrile hydratase) have been discovered so far. Various methods are known as methods of production of acrylamide using a microbial catalyst (see Patent Documents 1 to 8, for example).
On the other hand, since acrylonitrile has lower solubility in water or an aqueous acrylamide solution (7.3 g/100 g of water, 25° C.), in cases where dissolution of acrylonitrile in water is insufficient, adverse effects are caused, such as decreased catalyst productivity or deterioration of a microbial catalyst due to insufficient contact between acrylonitrile and the microbial catalyst, and increased loss of acrylonitrile by evaporation into the gas phase. The solubility of acrylonitrile in water or an aqueous acrylamide solution can be enhanced by vigorously stirring the aqueous medium or the reaction liquid. However, such vigorous stirring may damage the microbial catalyst, resulting in decreased activity.
In order to solve these problems, for example, Patent Document 1 describes that it is appropriate to add acrylonitrile or methacrylonitrile dropwise to the reaction system with stirring such that the substrate is always in the dissolved state in the reaction system. Further, Patent Document 2 discloses a method for producing an amide compound from a nitrile compound using a biocatalyst whose production cost and environmental load are suppressed, wherein contact and dispersibility of a nitrile compound and a biocatalyst are improved by setting the stirring power requirement to 0.08 to 1.3 kW/m3. Further, Patent Document 3 discloses an apparatus for producing an aqueous acrylamide solution, which apparatus comprises a reactor having a circulation route equipped with a pump, in which a part of the reaction mixture is circulated by the pump and at least one heat exchanger is provided. As a best mode, addition of acrylonitrile to the circulation route equipped with a pump is disclosed in the document. Further, Patent Document 4 describes that it is important to use an appropriate mixing device such as a rotor or line mixer to sufficiently mix the aqueous medium phase and the nitrile phase, which are separated into two layers when these are left to stand.
However, as can be seen in the above Patent Documents, prevention of both evaporation of acrylonitrile into the gas phase and damaging of the catalyst were still insufficient in the conventional techniques. Further, the method in Patent Document 3 requires much energy for producing power, and also requires additional cost for removal of heat due to possible generation of circulating heat by the pump. Further, the microbial catalyst is likely to be damaged by a vortex flow generated by the pump, resulting in decreased activity, which is problematic. Thus, effective means in the biocatalyst method that allow prevention of both evaporation of acrylonitrile into the gas phase and damaging of the catalyst still need to be studied.